TO BE

이다 is the Korean for "to be".

It is an irregular verb, so there are two ways to conjugate it into its polite level.

The first one is -예요 (it is attached to the noun it is related to) and you use it with the words which end with a vowel (eg: 차, tea → 차예요, it's tea); the second one is -이에요, and you use it when the words ending with a consonant (eg: 물, water → 물이에요, it's water).

If you need to use it in a formal event, you'll use -입니다; if you need to use it in an informal context you'll say -야.

Eg: 학교, school

Formal level → 학교입니다

Polite level → 학교예요

Informal level → 학교야

REMEMBER THAT in the majority of times you get the subject from the context, so you could easily understand that the previous phrases' meaning was "it's a school", and not "I'm a school" or "you're a school".